### Please describe the problem.

It's unclear how to use `git union-merge` as described [here](https://git-annex.branchable.com/git-union-merge/).

### What steps will reproduce the problem?

Run `git union-merge`, yields `git: 'union-merge' is not a git command. See 'git --help'.`

No binary `git-union-merge` is shipped in the standalone tarball and distros also don't seem to ship it.

### What version of git-annex are you using? On what operating system?

Tried with (on a Manjaro box):

- git-annex-standalone-amd64.tar.gz	2023-10-09 14:21 	51M	 from [here](https://downloads.kitenet.net/git-annex/linux/current/) (the build is a month old, is that right?)
- `git-annex` in Manjaro repos
- `git-annex-standalone-nightly-bin` from AUR
- `nix-shell -p git-annex` (10.20230926 in nixos-unstable)

### Have you had any luck using git-annex before? (Sometimes we get tired of reading bug reports all day and a lil' positive end note does wonders)

git-annex rules and is a marvelous tool.

I wanted to try the union merging to resolve merge conflicts on non-annexed files. It's not ideal, but might be better than repeated `git annex assist|sync` eventually adding the merge conflict markers `<<<<<<<<<` and the like to the files, breaking things like `.gitattributes` syntax which in turn has more devastating lockup consequences...
